# @inbounds macro on sparse arrays

**URL:** <https://discourse.julialang.org/t/inbounds-macro-on-sparse-arrays/34810>\
**Category:** Performance\
**Created:** [February 18, 2020, 1:30pm UTC](https://discourse.julialang.org/t/inbounds-macro-on-sparse-arrays/34810 "2020-02-18T13:30:35Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![mlanghinrichs](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mlanghinrichs/32/50371_2.png) [@mlanghinrichs](https://discourse.julialang.org/u/mlanghinrichs)\
**Post date:** [February 18, 2020, 1:30pm UTC](https://discourse.julialang.org/t/inbounds-macro-on-sparse-arrays/34810/1 "2020-02-18T13:30:36Z")

</div>

Hi all,

the @inbounds macro can speed up the in-place editing of a standard array by a factor of two; however it has no performance gain for the same task when applied on a sparse matrix. Please see my test functions and @btime benchmarks below. Why is this? Am I using @inbounds wrong on the sparse array / Is this feature not (yet) implemented? / Is there a reason it should not be implemented? I’m happy for any advice.

```julia
using BenchmarkTools
using SparseArrays

# test array
const testarr = [1.0, 2.0, 3.0]

# test sparse array
const testsparr = spzeros(3, 1)
testsparr[1, 1] = 1.0
testsparr[2, 1] = 2.0
testsparr[3, 1] = 3.0

# test function on standard array
function test_inbounds(testarr)
    for i=1:50_000_000
        testarr[2] += 2.0
    end
    testarr
end
function test_inbounds2(testarr)
    for i=1:50_000_000
        @inbounds testarr[2] += 2.0
    end
    testarr
end

# test functions on sparse array
function test_inbounds_sparse(testsparr)
    for i=1:50_000_000
        testsparr[2, 1] += 2.0
    end
    testsparr
end
function test_inbounds2_sparse(testsparr)
    for i=1:50_000_000
        @inbounds testsparr[2, 1] += 2.0
    end
    testsparr
end
function test_inbounds3_sparse(testsparr)
    @inbounds for i=1:50_000_000
        testsparr[2, 1] += 2.0
    end
    testsparr
end

```

benchmark results

```julia
@btime test_inbounds(testarr) # 111.919 ms (0 allocations: 0 bytes)
@btime test_inbounds2(testarr) # 48.900 ms (0 allocations: 0 bytes)

@btime test_inbounds_sparse(testsparr) # 551.772 ms (0 allocations: 0 bytes)
@btime test_inbounds2_sparse(testsparr) # 576.797 ms (0 allocations: 0 bytes)
@btime test_inbounds3_sparse(testsparr) # 581.419 ms (0 allocations: 0 bytes)

```

Best

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<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [February 18, 2020, 2:10pm UTC](https://discourse.julialang.org/t/inbounds-macro-on-sparse-arrays/34810/2 "2020-02-18T14:10:19Z")

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@inbounds let the compiler remove bounds check  
[https://docs.julialang.org/en/v1/devdocs/boundscheck/index.html](https://docs.julialang.org/en/v1/devdocs/boundscheck/index.html)

sparse arrays  
[https://docs.julialang.org/en/v1/stdlib/SparseArrays/index.html](https://docs.julialang.org/en/v1/stdlib/SparseArrays/index.html)  
are stored as  
[Compressed Sparse Column (CSC) format](https://en.wikipedia.org/wiki/Sparse_matrix#Compressed_sparse_column_.28CSC_or_CCS.29)  
see also  
[http://netlib.org/linalg/html\_templates/node92.html#SECTION00931200000000000000](http://netlib.org/linalg/html_templates/node92.html#SECTION00931200000000000000)

Without looking into the code I would say, bounds check doesn’t make any sense when accessing entries in a sparse array/matrix, or in other words, accessing values from a sparse matrix with indices involves special code where bounds checks aren’t needed.

In a normal array the memory location of the indexed value is calculated from the index and the memory size of the array value type. Therefor you can point to memory outside of the array with indices e.g. larger than the length of the array. This is checked with bounds checking and this is what costs some performance. @inbounds switches these checks off in normal arrays. In a sparse array there is nothing to switch off.

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<div class="post-metadata">

**Author:** ![mlanghinrichs](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mlanghinrichs/32/50371_2.png) [@mlanghinrichs](https://discourse.julialang.org/u/mlanghinrichs)\
**Post date:** [February 18, 2020, 2:19pm UTC](https://discourse.julialang.org/t/inbounds-macro-on-sparse-arrays/34810/3 "2020-02-18T14:19:21Z")

</div>

ok that makes sense, thanks a lot!